Vacuum pump

ABSTRACT

An improved split flow vacuum pump is provided with at least two of compound pumps housed in a common housing comprising first, second, third and fourth inlets for receiving gas from a respective first to fourth chamber; said pumps are arranged such that their rotational axis are angled relative to each other in the housing, preferably perpendicular to each other.

CROSS-REFERENCE OF RELATED APPLICATION

This application is a Section 371 National Stage Application ofInternational Application No. PCT/GB2016/051408, filed May 16, 2016,which is incorporated by reference in its entirety and published as WO2016/193664 A1 on Dec. 8, 2016 and which claims priority of BritishApplication No. 1509386.7, filed Jun. 1, 2015.

FIELD

The invention relates to a vacuum pump and in particular to a vacuumpump for differentially evacuating a vacuum system.

BACKGROUND

In a differentially pumped scientific instrument system, such as a massspectrometer, a sample and a carrier gas are introduced for analysis.One such example is given in FIG. 1. With reference to FIG. 1, in such asystem there exists a high vacuum chamber 110 immediately followingfirst, (depending on the type of system) second, and third evacuatedinterface chambers 111, 112, 114. The first interface chamber is thehighest-pressure chamber in the evacuated spectrometer system and maycontain an orifice or capillary through which ions are drawn from theion source into the first interface chamber 111. The second, optionalinterface chamber 112 may include ion optics for guiding ions from thefirst interface chamber 11 into the third interface chamber 114, and thethird chamber 114 may include additional ion optics for guiding ionsfrom the second interface chamber into the high vacuum chamber 110. Inthis example, in use, the first interface chamber is at a pressure ofaround 1-10 mbar, the second interface chamber (where used) is at apressure of around 10⁻¹-1 mbar, the third interface chamber is at apressure of around 10⁻²-10⁻³ mbar, and the high vacuum chamber is at apressure of around 10⁻⁵-10⁻⁶ mbar. Differentially pumped vacuum systemmay have different pressures dependent on requirements.

The high vacuum chamber 110, second interface chamber 112 and thirdinterface chamber 114 can be evacuated by means of a compound vacuumpump 116. In this example, the vacuum pump has two pumping sections inthe form of two sets 118, 120 of turbo-molecular stages, and a thirdpumping section in the form of a Holweck drag mechanism 122; analternative form of drag mechanism, such as a Siegbahn or Gaedemechanism, could be used instead. Each set 118, 120 of turbo-molecularstages comprises a number (three shown in FIG. 1, although any suitablenumber could be provided) of rotor 119 a, 121 a and stator 119 b, 121 bblade pairs of known angled construction. The Holweck mechanism 122includes a number (two shown in FIG. 1 although any suitable numbercould be provided) of rotating cylinders 123 a and corresponding annularstators 123 b and helical channels in a manner known per se.

In this example, a first pump inlet 124 is connected to the high vacuumchamber 110, and fluid pumped through the inlet 124 passes through bothsets 118, 120 of turbo-molecular stages in sequence and the Holweckmechanism 122 and exits the pump via outlet 130. A second pump inlet 126is connected to the third interface chamber 114, and fluid pumpedthrough the inlet 126 passes through set 120 of turbo-molecular stagesand the Holweck mechanism 122 and exits the pump via outlet 130. In thisexample, the pump 116 also includes a third inlet 127 which can beselectively opened and closed and can, for example, make the use of aninternal baffle to guide fluid into the pump 116 from the second,optional interface chamber 112. With the third inlet open, fluid pumpedthrough the third inlet 127 passes through the Holweck mechanism onlyand exits the pump via outlet 130.

In this example, in order to minimise the number of pumps required toevacuate the spectrometer, the first interface chamber 111 is connectedvia a foreline 131 to a backing pump 132, which also pumps fluid fromthe outlet 130 of the compound vacuum pump 116. The backing pumptypically pumps a larger mass flow directly from the first chamber 111than that from the outlet 130 of the compound vacuum pump 116. As fluidentering each pump inlet passes through a respective different number ofstages before exiting from the pump, the pump 116 is able to provide therequired vacuum levels in the chambers 110, 112, 114, with the backingpump 132 providing the required vacuum level in the chamber 111. Thevacuum pumping arrangement shown in FIG. 1 is also described in forexample U.S. Pat. No. 5,733,104.

The performance and power consumption of the compound pump 116 isdependent largely upon its backing pressure, and is therefore dependentupon the foreline pressure (and the pressure in the first interfacechamber 111) offered by the backing pump 132. This in itself isdependent mainly upon two factors, namely the total mass flow rateentering the foreline 131 from the scientific instrument and the pumpingcapacity of the backing pump 132. Many compound pumps having acombination of turbo-molecular and molecular drag stages are onlyideally suited to relatively low backing pressures, and so if thepressure in the foreline 131 (and hence in the first interface chamber111) increases as a result of increased mass flow rate or a smallerbacking pump size, the resulting deterioration in performance andincrease in power consumption can be rapid. In an effort to increasemass spectrometer performance, manufacturers often increase the massflow rate into the spectrometer, thus requiring increased size or numberof backing pumps in parallel to accommodate for the increased mass flowrate. This increases costs, size (footprint) and power consumption ofthe overall pumping system required to differentially evacuate the massspectrometer.

The discussion above is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter. The claimed subject matter is notlimited to implementations that solve any or all disadvantages noted inthe background.

SUMMARY

One embodiment provides a vacuum pump for differentially pumping aplurality of chambers, the vacuum pump comprising: a housing whichhouses a plurality of compound pumping arrangements supported forindependent rotation one from another on respective drive shafts byseparate motors; a first housing inlet for receiving fluid from a firstchamber; a second housing inlet for receiving fluid from a secondchamber; a first of the compound pumping arrangements comprising a firstpumping section comprising a turbo molecular pumping mechanism and asecond pumping section downstream from the first pumping section, thesections being arranged such that fluid entering the compound pump fromthe first inlet passes through the first and second pumping sections andfluid entering the compound pump from the second inlet passes through,of said sections, only the second section; the vacuum pump furthercomprising: a third housing inlet for receiving fluid from a thirdchamber; a fourth housing inlet for receiving fluid from a fourthchamber; a second of the compound pumping arrangements comprising athird pumping section comprising a turbo molecular pumping mechanism anda fourth section downstream from the first pumping section, the sectionsbeing arranged such that fluid entering the second compound pumpingarrangement from the third inlet passes through the third and fourthpumping sections and fluid entering the second compound pump from thefourth inlet passes through, of said sections, only the fourth section.

In another aspect there is provided a vacuum pump for differentiallypumping a plurality of chambers at different pressures, the vacuum pumpcomprising: a housing comprising first and second housing inlets forconnection to respective vacuum chambers, the housing comprising a borefor receiving a cartridge casing of a compound vacuum pumpingarrangement, the compound vacuum pumping arrangement comprising a firstpumping section and a second pumping section downstream from the firstpumping section, the sections being arranged such that fluid enteringthe compound pump from the first housing inlet passes through the firstand second pumping sections and fluid entering the compound pumpingarrangement from the second inlet passes through, of said sections, onlythe second section, the cartridge casing comprising a first fluid inletarrangement exposed to receive fluid from the first housing inlet whenthe cartridge is located in the housing and a second fluid inletarrangement exposed to receive fluid from the second housing inlet whenthe cartridge is located in the housing, the first fluid inletarrangement comprising at least one radial fluid inlet for receivingfluid in a generally radial direction into the casing and at least oneaxial fluid inlet for receiving fluid in a generally axial directioninto the casing.

The second fluid inlet arrangement may be formed in a portion of thecartridge casing which protrudes into a volume in gas communication withthe first housing inlet and the volume may encircle an axis of thecompound pumping arrangement.

The first fluid inlet arrangement may comprise a plurality of saidradial inlets spaced about the circumference of the casing and exposedto the volume.

Said at least one axial fluid inlet may be formed at least in part by abearing mount supporting a bearing of the compound pumping arrangementand supported by the cartridge casing. The bearing mount, or spider, maybe received in the volume.

At least one turbo molecular stage (or array of rotor blades) may belocated at the axial fluid inlet for drawing gas through the inlet fromthe volume.

The first section of the compound vacuum pumping arrangement may bespaced from the spider or said at least one turbo molecular stage by anamount generally equal to or greater than an axial width of the radialfluid inlets.

Other preferred and/or optional aspects of the invention are defined inthe accompanying dependent claims.

The Summary is provided to introduce a selection of concepts in asimplified form that are further described in the Detail Description.This summary is not intended to identify key features or essentialfeatures of the claimed subject matter, nor is it intended to be used asan aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the embodiments may be well understood, an embodimentthereof, which is given by way of example only, will now be described inmore detail, with reference to the accompanying drawings, in which:

FIG. 1 is a simplified view of a prior art vacuum pumping arrangementand differentially pumped vacuum system;

FIG. 2 is a perspective view of a vacuum pump;

FIG. 3 is a further perspective view of the vacuum pump shown in FIG. 2;

FIG. 4 shows a section of the vacuum pump taken along a centrallongitudinal and vertical plane;

FIG. 5 is a perspective view of the vacuum pump prior to full assembly;and

FIG. 6 is a part section for showing a cartridge casing in the vacuumpump.

DETAILED DESCRIPTION

Referring to FIG. 2, a vacuum pump 10 is shown for differentiallypumping a plurality of chambers. As described above in relation to theprior art, the chambers may form part of a mass spectrometer system andcomprise a high vacuum chamber 110 immediately following first,(depending on the type of system) second, and third evacuated interfacechambers 111, 112, 114. Alternatively, the vacuum system may comprise aplurality of chambers as disclosed in more detail in US2015/0056060.

The vacuum pump 10 comprises a housing 12. The housing 12 may be cast ormachined from a suitable metallic material, such as steel or iron, andhas a one-piece construction for housing the various pumping mechanismsof the pump. In this example the housing is aluminium and machined froma solid block, but could be extruded or cast. Aluminium is preferredbecause it is light-weight. The housing comprises a plurality of housinginlets 14, 16, 18, 20, 22, 24 and can be fixed to the vacuum system bysuitable fastening members and sealed to allow fluid communicationbetween the different pressure chambers of the system and selectedpumping mechanisms of the pump. The fluid inlets are surrounded bygrooves 26 for receiving sealing members (not shown), such as o-rings,for sealing against the vacuum system to avoid or resist the leakage ofambient air into the pump. Inlets 18, 20, 22, 24 are surrounded by asingle groove and sealing member.

The housing comprises a plurality of flow paths which guide fluid fromthe housing inlets towards the inlets of the different pumpingmechanisms, as shown in more detail in FIG. 4. These flow paths areformed by the internal structure of the housing and may be cast in themanufacturing process or machined into the housing.

The vacuum pump comprises two vacuum pumping arrangements 28, 30. Inthis example, vacuum pumping arrangement 30 is arranged to evacuatepressure chambers through housing inlets 18, 20, 22, 24 at a relativelyhigher vacuum (lower pressure) compared to vacuum pumping arrangement 28which is arranged to evacuate pressure chambers through housing inlets14, 16 at a relatively lower vacuum (higher pressure). The vacuumpumping mechanisms for the two arrangements are housed within thehousing 12. In each case, part of the vacuum pumping arrangementsextends externally to the housing and these external parts may includethe motor and electrical connections for connecting the arrangements toa source of electrical power. Locating the motors at partially externalto the housing allows heat generated during use to escape from the pumpmore easily.

FIG. 3 shows the vacuum pump 10 from a different perspective, underneaththe pump. The first vacuum pumping arrangement 30 comprises an inletport 32 and the second vacuum pumping arrangement 28 comprises anexhaust port 34. A foreline pipe 36 connects the exhaust port with theinlet port for fluid communication. As described in more detail belowgas is exhausted from the second vacuum pumping arrangement through theforeline to the first vacuum pumping arrangement, where it is pumped byat least one pumping mechanism. The first vacuum pumping arrangementcomprises an exhaust port (not shown in this Figure) for exhausting gasthrough another foreline pipe to a separate backing, or primary, pump.In this way, the second vacuum pumping arrangement is backed in seriesby the first vacuum pumping arrangement and a primary pump, and thefirst vacuum pumping arrangement is backed by the primary pump. This canreduce the overall power consumption of the system or of the firstvacuum pumping arrangement 28. Alternatively the reduced backingpressure could be used to enhance the overall compression of arrangement28.

In an alternative, the housing could be configured to include internalstructure defining the flow path between the vacuum pumpingarrangements, thereby eliminating the need for additional fittings andpipes. In other examples, the vacuum pump can be backed independently bya single or multiple backing pumps.

FIG. 4 shows a section through the vacuum pump 10. The first vacuumpumping arrangement is inserted into a bore 38 of the housing 12 throughan opening 40 in the underside of the housing and fastened in positionwith fastening members (not shown). An o-ring 42 seals the arrangementwhen in position. The first vacuum pumping arrangement comprises in thisexample seven vacuum pumping sections each having a pumping mechanismand one or more pumping stages. The number of sections, stages in eachsection and type of pumping mechanism may be selected as requireddepending on pumping requirements, such as capacity and compression. Thearrangement 30 comprises a drive shaft 58 supported for rotation byupper and lower bearings 60, 62. Typically the upper bearing is amagnetic bearing (with a back-up bearing) and the lower bearing is aroller bearing. The upper bearing is supported by a spider 63 having acentral hub from which three arms extend in a radial direction. The armsare fixed to the housing to provide support for the bearing and thevacuum pumping arrangement, whilst allowing space for gas to enter themost upstream pumping mechanism 44. The spider could also be machinedinto and form part of the housing (i.e. integral with the housing).

A motor 64 drives rotation of the drive shaft and is connected to asource of electrical power. The first pumping arrangement is a highspeed pump and is typically rotated at speeds of between about 10,000and 100,000 rpm.

The sections 44, 46, 48, 50 of vacuum pumping arrangement 30 eachcomprise a turbo molecular pumping mechanism. These sections compriserespectively four stages, three stages, three stages and two stages, butmore or fewer stages may be provided as required. Although at least onesection comprising a turbo molecular pumping mechanism is required togenerate the required vacuum pressure, the other sections may bereplaced with other types of pumping mechanism. Sections 52 and 54 areeach four stage drag pumping mechanisms, and section 56 is a two stageregenerative pumping mechanism. The regenerative pumping mechanism isotherwise known as an aerodynamic mechanism in which an array of bladeson a rotor disc extend into respective channels of a stator generating avortex in the channels on rotation which compress the gas being pumped.Different types of pumping mechanisms may be used in the lattersections, as well as different numbers of stages, depending on pumpingrequirements.

The first vacuum pumping mechanism comprises four fluid inlets to thevarious sections. Section 44 has a fluid inlet 66 connected for fluidcommunication with the housing inlet 24 by internal flow path 67.Section 46 has a fluid inlet 68 connected for fluid communication withthe housing inlet 22 by internal flow path 69. Section 48 has a fluidinlet 70 connected for fluid communication with the housing inlet 20 byinternal flow path 71. Section 50 has a fluid inlet 72 connected forfluid communication with the housing inlet 18 by internal flow path 73.Flow paths 69, 71, 73 extend away from the housing inlets and through 90degrees to their respective fluid inlets, but may alternatively bedirect in line with the pumping mechanism (i.e. not through 90 degrees).

Fluid entering through housing inlet 24 passes through all of thepumping sections of the first vacuum pumping arrangement, namelysections 44 to 56. Fluid entering through housing inlet 22 passesthrough sections 46 to 56 only. Fluid entering through housing inlet 20passes through sections 48 to 56 only and fluid entering through housinginlet 18 passes through sections 50 to 56 only. In this example, housinginlet 24 is evacuated to the lowest pressure and the evacuation pressuregradually increases as gas passes through fewer sections.

Pumping sections 52, 54, 56 provide a backing pressure for the turbomolecular sections of the first vacuum pumping arrangement 30, since aturbo molecular mechanism cannot, or at least cannot efficiently,exhaust at atmosphere. Whilst in some examples the most downstreamsection 56 may exhaust at atmosphere, typically it exhausts belowatmosphere and is itself backed by a separate primary pump, oralternatively another similar turbo molecular pump then a primary pump.

As discussed above, the exhaust 34 of the second vacuum pumpingarrangement 28 is connected to an inlet 32 of the first vacuum pumpingarrangement 30. The section of FIG. 4 does not show these ports, howeverthe exhaust 34 is located downstream of the most downstream pumpingsection of the second arrangement and the inlet 32 is located upstreamof at least one of sections 52, 54, 56 and downstream of at leastsection 50. In this way, the exhaust 34 is backed by one, two or threepumping sections. The backing section is preferably configured as abooster mechanism whereby the compression ratio of the section isbetween 10:1 and 1:1 (for example) so that pumping capacity isincreased. A section configured as a booster is capable of pumping agreater amount of gas which is useful in the event that a large amountof gas is input to the mass spectrometer.

The second vacuum pumping arrangement 28 comprises a cartridge in thisexample, although a cartridge envelope is not essential for arrangement28 (or both pumps) and alternatively it may be integrated directly intothe housing. The cartridge 75 comprises a casing 74 for supporting thepumping mechanisms of the cartridge. The casing is configured so thatthe cartridge can be inserted into and engage with a bore 76 of thehousing 12 to expose fluid inlets 78, 80 of the pumping mechanisms torespective housing inlets 16, 14. FIG. 5 shows a perspective view of thevacuum pump 10 with the cartridge prior to insertion in bore 76 throughhousing opening 82.

The inner surface of the bore 76 guides the cartridge 75 towards thefully inserted position shown in FIG. 4. The casing has an outwardlyprojecting flange 83 which forms with housing end surface 84 abutmentsurfaces for locating the cartridge in the correct position and limitingthe extent to which the cartridge 75 can be inserted into the housing12. Fastening members 86 fix the cartridge in position when it has beeninserted. The members engage in closed bores 88 of the housing,typically by threaded engagement.

As shown in FIG. 4, the housing 12 is shaped so as to expose the bore ata number of locations to allow fluid to enter the fluid inlets 78, 80when the cartridge 14 is in the fully inserted position. Flow paths 79,81 manufactured in the housing guide flow from the housing inlets 14, 16to respective fluid inlets 78, 80 of the cartridge. The housing alsodefines a volume 89 for gas flow from housing inlet 16. Gas from inlet16 may therefore pass into the cartridge radially through fluid inlet 78or axially through a fluid inlet 85. The provision of radial and axialfluid inlets increases conductance into the pumping arrangement. It ispreferable in this case as shown that at least one turbo molecularpumping stage 87 (or array of rotor blades) is located at the fluidinlet 85 to draw gas into the cartridge. The provision of two fluidinlets at the housing inlet 16 produces low conductance resistance toflow.

In more detail and as shown additionally in FIG. 6, the cartridge casingdefines a plurality of apertures 91 spaced about its circumferenceforming radial fluid inlets into the vacuum pumping arrangement 28. Fourapertures 91 are provided in this example at 90 degrees to one anotherbut other configurations are possible. The apertures are located in partof the housing that extends into volume 89 and therefore all aperturesare exposed to the volume, including the apertures located furthest fromthe housing inlet 16. The array of rotor blades 87 and the pumpingsection 90 are spaced apart in an axial direction by an amount which isapproximately equal to the axial width of the apertures 91. Thisconfiguration allows gas to penetrate into the space 93 between thepumping mechanisms towards the drive shaft to enable pumping by bothradially outer and radially inner portions of the turbo molecularmechanism 90. Without such a space 93 the gas would interact only withthe radially outer portions of the mechanism, or to a much greaterextent than with the radially inner portions, and less efficient pumpingwould be achieved.

The fluid inlet 85 of the cartridge casing is formed by a generallycircular aperture which is open in the axial direction (to the right inFIGS. 4 and 6). The casing comprises a shoulder portion 95 which islocated upstream of the apertures 91 and fully within volume 89. Theshoulder portion seats a bearing mount, or spider, 97 for mountingbearing 98 in position, which is likewise received in volume 89. Thespider comprises an outer rim fixed to the casing, an inner hub forsupporting the bearing and three radial arms (or spokes) connecting therim with the hub. This configuration provides three apertures extendingthrough approximately 120 degrees forming the axial fluid inlet 85 intothe vacuum pumping arrangement 28. The spider may have fewer or moreradial arms as is not restricted to three.

The cartridge casing further comprises apertures 99 forming a fluidinlet to the pumping section 92. Unlike apertures 91 located in volume89, only that part of the casing proximate the housing inlet 14 isexposed to receive gas and therefore apertures 99 are provided only inan upper part of the casing.

The pumping arrangement as shown fits horizontally into the housing, butcould be arranged vertically—similarly the first arrangement could behorizontal—hence providing four configurations. That ishorizontal-horizontal, horizontal-vertical, vertical-horizontal, andvertical-vertical.

In this example, the second vacuum pumping arrangement 28 comprisesthree pumping sections 90, 92, 94. Section 90 comprises three turbomolecular stages. Section 92 comprises three Holweck stages. Section 94comprises two Seigbahn stages. Different types of molecular drag orother pumping mechanisms may be used in sections 92 and 94, or there mayonly two sections in total. The number of stages in the sections may bevaried as required dependent on pumping requirements.

A drive shaft 96 supports the pumping mechanisms for rotation and isitself supported by bearings 98, 100. In this example, bearing 98 is adry magnetic bearing with roller bearing back-up and bearing 100 is alubricated roller bearing. The drive shaft is driven by motor 102connected to a source of electrical power at high rotational speeds forexample between about 10,000 and 100,000 rpm.

Rotation of the pumping sections 90, 92, 94 causes gas to flow fromhousing inlet 16 through fluid inlets 78, 85 passing through section 90(and turbo molecular stage 87), section 92 and section 94. Gas is causedto flow from housing inlet 14 through fluid inlet 80 passing through, ofthe sections, only the downstream sections 92, 94. The pressure chambersin fluid communication with the housing inlets 14, 16 are evacuated atdifferent pressures; housing inlet is evacuated at a lower pressurebecause gas passing through it is conveyed through more pumping sectionsand section 90 is a configured to pump at lower pressures. More than twohousing inlets may be evacuated by the cartridge.

Gas is exhausted from the cartridge 75 through exhaust port 34 andpasses through a booster section (one or more of sections 52, 54, 56) ofthe first vacuum pumping arrangement prior to a separate backing orprimary pump. If the first pump power is high the second pump could beused to back the first (switching the backing arrangement shown).

The cartridge type configuration of the second vacuum pumpingarrangement 28 is advantageous in that it can readily be removed andinserted into the housing, allowing easy maintenance, repair orreplacement. Since the casing of the cartridge provides the supportrequired for operation (and rotation at high speeds) the vacuum pump 10can be more easily manufactured and assembled. The various components ofthe cartridge are assembled outside the housing 12, without the need tomanufacture typically intricate structures for supporting the mechanisminside the housing. In the example of vacuum pump 10, vacuum pumpingarrangement 30 does not have a cartridge type configuration and cannotbe operated without support inside the housing, particularly withoutspider 63 and bearing assembly 60. However, vacuum pumping arrangement30 may alternatively be formed as a cartridge type configurationsimilarly to vacuum pumping arrangement 28.

The vacuum pumping arrangements may have drive shafts having respectiveaxes of rotation which are angled one relative to another. In theexample shown the drive shafts 58, 96 have first and second axes ofrotation, and the first axis is perpendicular to the second axis. Theaxis 96 intersects axis 58, but in other examples the axes may beoffset. In an earlier document of the present applicant (EP2273128) anaxis of a cartridge type vacuum pumping arrangement is inclined by anangle θ to the direction of gas flow through the pressure chambers of adifferentially pumped system and in further embodiments a similarconstruction can be adopted whereby a pumping axis of one pumpingarrangement is at an angle θ as is the case with the earlier document.

As shown in the Figures the axis 58 is vertical and the axis 96 ishorizontal, with respect to gravitational force. It is preferable in thepresent construction that the axis 58 is vertical in order to cancel outthe effect of gravity acting on the moving parts (rotor parts) of thepumping arrangement. Particularly, upper bearing 60 is a non-contactbearing and therefore allows a small amount of radial movement of therotor (limited by the back-up bearing). Therefore the drive shaft can toan extent be considered cantilever, but as gravitational force islargely cancelled and does not cause significant radial movement. Itwill also be appreciated that there are many pumping sections, seven intotal and four turbo molecular sections (requiring high speed rotation),thereby contributing to the overall length of the drive shaft. Hence instructures with a multiplicity of pumping sections (more than four) or amultiplicity of turbo molecular sections (more than two) it isdesirable, but not essential, that the drive shaft is vertical forimproved rotor dynamics.

The drive shaft 96 of the second vacuum pumping arrangement ishorizontal, because the problems explained above in relation to thefirst vacuum pumping arrangement are less apparent, and a horizontalorientation is advantageous for conserving foot-print or pump size. In aone-piece housing construction there is also the issue of mass as thevacuum pump is typically mounted by suspending it from the underside ofa vacuum system. A horizontal drive shaft reduces the size of thehousing and also its mass.

As described above in detail with reference to the Figures, the housing12 houses a plurality of compound pumping arrangements 28, 30 supportedfor independent rotation one from another on respective drive shafts 58,96 by separate motors 64, 102. Independent rotation and drive allowimproved versatility for application to multiple different pumpingrequirements. For example, operators of mass spectrometer systems areincreasing the sample size of gas introduced to upstream pressurechambers of the system and accordingly there may be a cyclical ortransient requirement during operation for the second vacuum pumpingarrangement to pump large quantities of gas. In these circumstances, thepower drawn from the motor 102 is increased to overcome increasedresistance to rotation in sections 90, 92, 94. However, the first vacuumpumping arrangement 30 is independent and continues operation withoutinterruption or increased power requirement.

The power supply for the vacuum pumping arrangements 28, 30 may beshared so that electrical power is supplied to the motors 64, 102 by asingle supply unit (not shown). In this case, the unit may provide theelectrical power and control of the units may be performed at thepumping arrangement or in the unit. That is two frequency converters maybe provided in the unit or one in each of the pumping arrangements. Theadvantage of a single supply unit is that the requirement for electricalpower is spread across or shared by both pumping arrangement so if onepumping arrangement is subjected to high load conditions and the otherto low load conditions the overall power consumption does not increase.Such conditions may occur for example if both vacuum pumpingarrangements are operating at ultimate pressure and a sample gas isintroduced to the vacuum system—the pressure and amount of gas in theupstream chambers increases, with a commensurate increase in the load onthe second vacuum pumping arrangement 28, but there is a delay prior toincreased pressure in the downstream chambers and increased load on thefirst vacuum pumping arrangement 30. Alternatively, each vacuum pumpingarrangement can be provided with a dedicated source of electrical power.

In some known vacuum pumping systems, a plurality of pumps are providedin respective housings for differentially evacuating a vacuum system.However, in these known systems the pumps are arranged so that gasexhausted from a lower pressure pump is conveyed to an inlet of thehigher pressure pump. Therefore the pumps are arranged in series. Invacuum pump 10, however, the two vacuum pumping arrangements are not inseries and instead evacuate the pressure chambers in parallel. A seriesrelationship is partly provided but only between the exhaust of thesecond vacuum pumping arrangement and the booster section 52, 54, 56.

The housing 12 is manufactured to include the required internalstructure both for seating the vacuum pumping arrangements in thecorrect positions and for guiding fluid flow from the housing inlets tothe fluid inlets of the vacuum pumping arrangements. The internalstructure is configured to have a relatively simple shape which permitsformation by casting or moulding without the requirement for extensivesubsequent machining. The internal structure is configured to include aplurality of shaped flow conduits defined by internal partitioning wallsfor directing flow to the fluid inlets of both vacuum pumpingarrangements. The provision of a single one-piece housing for directinggas flow removes the requirement for forelines and other pipeworkthereby avoiding the use of additional components, the time required forassembly and a potential cause of leakage.

Although elements have been shown or described as separate embodimentsabove, portions of each embodiment may be combined with all or part ofother embodiments described above.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are described asexample forms of implementing the claims.

1. A vacuum pump for differentially pumping a plurality of chambers, thevacuum pump comprising: a housing which houses a plurality of compoundpumping arrangements supported for independent rotation one from anotheron respective drive shafts by separate motors; a first housing inlet forreceiving fluid from a first chamber; a second housing inlet forreceiving fluid from a second chamber; a first of the compound pumpingarrangements comprising a first pumping section comprising a turbomolecular pumping mechanism and a second pumping section downstream fromthe first pumping section, the sections being arranged such that fluidentering the compound pump from the first inlet passes through the firstand second pumping sections and fluid entering the compound pump fromthe second inlet passes through, of said sections, only the secondsection; the vacuum pump further comprising: a third housing inlet forreceiving fluid from a third chamber; a fourth housing inlet forreceiving fluid from a fourth chamber; a second of the compound pumpingarrangements comprising a third pumping section comprising a turbomolecular pumping mechanism and a fourth section downstream from thefirst pumping section, the sections being arranged such that fluidentering the second compound pumping arrangement from the third inletpasses through the third and fourth pumping sections and fluid enteringthe second compound pump from the fourth inlet passes through, of saidsections, only the fourth section.
 2. The vacuum pump as claimed inclaim 1, wherein the drive shafts have respective axes of rotation whichare angled one relative to another.
 3. The vacuum pump as claimed inclaim 1, wherein the drive shafts have first and second axes ofrotation, and the first axis is perpendicular to the second axis.
 4. Thevacuum pump as claimed in claim 1, wherein at least one of the pumpingarrangements is in the form of a cartridge comprising a casing forsupporting the pumping mechanisms of the cartridge and configured sothat the cartridge can be inserted into and engage with a bore of thehousing to expose fluid inlets of the pumping mechanisms to respectivehousing inlets.
 5. The vacuum pump as claimed in claim 1, wherein thesecond and fourth sections of the compounds pumping arrangementscomprise a molecular drag pumping mechanism or regenerative pumpingmechanism.
 6. The vacuum pump as claimed in claim 1, wherein the secondsection of the first pumping arrangement comprises a booster pumpingmechanism and the housing comprises a booster inlet arranged forconnection to an exhaust of the second vacuum pumping arrangement sothat the second section backs the second vacuum pumping arrangement andthe first section of the first vacuum pumping arrangement.
 7. The vacuumpump as claimed in claim 6, wherein the housing comprises an exhaustforming an outlet from the booster pumping mechanism for connection to abacking pump so that the backing pump can back the first pumpingarrangement and the second pumping arrangement.